In a scanning optical unit used in an electrophotographic image forming apparatus, light emitted from a light source is collimated into a light beam with a collimating lens, and this light beam is deflected in a main scanning direction by rotating reflecting surfaces of an optical deflector, and then the deflected light beam is focused on a photoconductor drum through a scanning lens. An incident optical system provided between the light source and reflecting surfaces of the optical deflector causes the light beam to converge in a sub-scanning direction and to be focused on the reflecting surfaces, whereas a scanning optical system provided between the optical deflector and the photoconductor drum causes the light beam reflected by the reflecting surfaces to converge in the main scanning direction and in the sub-scanning direction to form an image.
There is conventionally known a scanning optical unit, in which a ratio of a lateral magnification βs in the sub-scanning direction to a lateral magnification βm in the main scanning direction is specified.